Large-scale networks of coupled oscillators can be inherently difficult to understand due to complex and diverse interactions. Recent developments in connectivity analysis and graphical network theory provide effective ways to probe these interactions. To date, these tools have been widely applied within the field of cognitive neuroscience; however their use has been limited in the study of neural networks with single neuron resolution. The suprachiasmatic nucleus (SCN) is an excellent system where we may now use these methods to probe the functional connectivity of a neural network and reveal organizing principles in populations of coupled oscillators. The SCN is composed of approximately 20,000 neurons which express daily rhythms in firing rate and gene expression. Coupling between oscillating neurons is necessary to maintain synchronous activity and produce a coherent daily output which regulates circadian rhythms including metabolism, hormone release, and sleep-wake cycles. Disruption of these daily rhythms has deleterious effects on many aspects of human physiology including cognition and mood. To understand circadian regulation within the brain, we must understand the intrinsic properties of each SCN oscillator and how these properties are modulated by network interactions. Revealing how these oscillators couple to generate a coherent rhythmic output will have important clinical implications for prevention and treatment of circadian rhythm disorders, including mood disorders. In Specific Aim #1, we will determine how network interactions alter the circadian behavior of individual SCN neurons. Using state-of-the-art multielectrode array and single-cell imaging technologies in conjunction with genetic and pharmacologic manipulations aimed at blocking specific neurotransmitter pathways, we propose to compare firing rate and gene expression patterns in individual SCN neurons from networks where the native connections are left largely intact (explanted slices) or networks where neurons have been forced to regrow in culture (high density dispersals). We will test the roles of several neurotransmitters in modulating the intrinsic properties of these neurons. In Specific Aim #2, we will analyze the functional connectivity within these same cultures by measuring correlated activity and mapping communication between SCN neurons over milliseconds to days. We will test the relative roles of different neurotransmitters in mediating this functional connectivity and circadian synchrony. Together, these experiments will elucidate the neural circuits which regulate the amplitude, period, synchrony and precision of circadian rhythms within the mammalian SCN. The proposed experiments have the potential to transform our understanding of SCN connectivity and more generally contribute to our knowledge of complex systems and networks of coupled oscillators. By probing the dynamics of a biologically important network, we will gain enhanced insight into how SCN neurons synchronize; and develop a better understanding of how these networks may be modulated to treat circadian rhythm disorders including mood disorders. 1